1. Field of the Invention
This invention relates to non-invasive approaches for determining cardiac output in patients, specifically to partial re-breathing techniques for determining cardiac output in patients, and most particularly to airway valves for initiating and terminating extension of the respiratory path volume, as well as ventilator circuits so equipped.
2. Statement of the Art
It is desirable, or even essential, to determine or monitor the cardiac output of a patient in many medical and surgical procedures. Invasive techniques well known and used in the art employ the use of catheters inserted at certain arterial points (e.g., femoral artery, jugular vein, etc.) to monitor blood temperature and pressure in order to determine cardiac output of the patient. Although capable of producing reasonably accurate results, the invasive nature of such procedures, with the attendant trauma and risk of infection, has demonstrated an unreasonably high potential for morbidity and mortality consequences.
Adolph Fick's measurement of cardiac output, first proposed in 1870, has served as the standard by which all other means of determining cardiac output have been evaluated since that date. Fick's well-known equation, written for CO.sub.2, is: ##EQU1## where Q is cardiac output, V.sub.CO2 is the amount of CO.sub.2 excreted by the lungs and and are the arterial and venous CO.sub.2 concentrations, respectively. Notably, the
Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output because the arterial and mixed venous blood must be sampled in order to determine arterial and venous CO.sub.2 concentrations.
It has previously been shown, however, that non-invasive techniques may be used for determining cardiac output while still using principles embodied in the Fick Equation. That is, expired CO.sub.2 ("pCO.sub.2 ") levels can be monitored to estimate arterial CO.sub.2 concentrations and a varied form of the Fick Equation can be applied to evaluate observed changes in pCO.sub.2 to estimate cardiac output. One use of the Fick Equation to determine cardiac output in non-invasive procedures requires the comparison of a "standard" ventilation event to a sudden change in ventilation which causes a change in expired CO.sub.2 values and a change in excreted volume of CO.sub.2. One commonly practiced means of providing a sudden change in effective ventilation is to cause the ventilated patient to re-breath a specified amount of previously exhaled air. This technique has commonly been called "re-breathing."
Prior methods of re-breathing have used the partial pressure of end-tidal CO.sub.2 to approximate arterial CO.sub.2 while the lungs act as a tonometer to measure venous CO.sub.2. Such an approach to re-breathing has not proven to be satisfactory for determining cardiac output because the patient is required to breath directly into and from a closed volume in order to produce the necessary effect. However, it is usually impossible for sedated or unconscious patients to actively participate in inhaling and exhaling into a bag. The work of some researchers has demonstrated that the Fick Equation could be further modified to eliminate the need to directly calculate venous P.sub.CO.sbsb.2 (P.sub.VCO.sbsb.2) by assuming that the P.sub.VCO.sbsb.2 does not change within the time period of the perturbation- an assumption that could be made by employing the partial re-breathing method. (See, Capek et al., "Noninvasive Measurement of Cardiac Output Using Partial CO.sub.2 Rebreathing", IEEE Transactions On Biomedical Engineering, Vol. 35, No. 9, Sep. 1988, pp. 653-661.)
Known partial re-breathing methods are advantageous over invasive measuring techniques because they 1) are non-invasive, 2) use the accepted Fick principle of calculation, 3) are easily automated, 4) require no patient cooperation and 5) allow cardiac output to be calculated from commonly monitored clinical signals. Thus, non-invasive cardiac output (NICO) techniques are rapidly gaining favor.
However, portions of known apparatus (i.e., ventilator circuits) used for partial re-breathing techniques employed in NICO, such as airway valves for initiating and terminating an extension of a patient's respiratory path through a conduit (tubing) loop or other reservoir, are of somewhat complex, relatively expensive construction, which render these somewhat contamination-prone and difficult to sterilize devices too expensive to be used as disposable units. In addition, conventional airway valves may unacceptably increase respiratory path volume when in normal operating (non-re-breathing) mode, may exhibit unduly high resistance to air flow in the normal operating mode, may require undesirably high energy levels to actuate, may not return in a fail-safe manner to the normal operating mode if actuation energy is removed, and their physical configurations may render them susceptible to malfunction responsive to the presence of moisture and other contaminants typically found in an airway circuit in close proximity to the patient. Finally, conventional airway valve designs may provide, or dictate, a fixed re-breathing volume, which fixed volume may not be optimum, or even suitable, for patients of various sizes and respiratory capacities.
Thus, it would be advantageous to provide a relatively simple and inexpensive, reliable, easy to fabricate, one-use (disposable) airway valve of a design which prevents cross-contamination between patients, minimizes any significant increase in respiratory path volume or air flow resistance therethrough when in a normal operating mode so as to not interfere with the function of the associated ventilator circuit, is usable with state-of-the art ventilator circuits without modification thereto and in a manner which is easy to actuate and control with minimal modifications to existing monitors, operates in a fail-safe manner so as to default to the normal operating mode, is resistant to contaminant-induced malfunctions, and easily accommodates variation in re-breathing volumes. Furthermore, it would be desirable for such an airway valve to introduce only minimal additional equipment bulk and weight in the vicinity of the patient.